It’s funny you bring up Union vs Confederacy, because Reconstruction should be a blueprint of what not to do. We’re still paying today for the failures of our forefathers to not have the bravery to stamp out the worst cultural elements of the Confederacy, let alone the extended century-long suffering of formerly enslaved peoples in what ranged from “separate but equal” to outright apartheid.

So yes, it was very much the Union’s fault for not properly cutting the many heads of the hydra with great prejudice.

Edit: the Nuremberg trials are a fantastic blueprint for what to do, though. And that’s what was missing after the Civil War. The sentences handed out at Nuremberg aren’t necessarily relevant to the crimes committed by those in power today, but the cultural shift that materialized in Germany post-war is the type of cultural shift needed to counteract the damage of the current administration: against corruption, grift, and executive overreach.